In recent years, the use of resins for parts not only in the automobile industry but also in various kinds of industrial machines has become noteworthy, for many reasons such as lighter weights, cost reductions, less friction, or recycling. But many new issues have arisen as the constituent elements of the parts have diversified, and various technical improvements are being undertaken.
For example, there are lubrication points such as the sliding parts of movable elements in electric door mirrors of cars or telescopic steering columns, various sliding parts such as the rack guides of R & P steering, the sliding parts of drive transmission gears, actuators and air cylinders in steering gears, linear guides or ball screw retainers of machine tools or bearing retainers, the sliding parts of crane booms, and also resin gear parts in acoustical equipment such as radio cassette players, video tape recorders and CD players, the resin gear parts in office automation equipment such as printers, photocopiers and fax machines, and the sliding parts of various kinds of electrical switches, which function through contact between materials other than metallic materials such as resin and resin or resin and metal.
In the prior art, in the field of lubrication, because virtually all the constituents of machines have been of metallic materials, there has been a long and broad history of research on friction and wear of pairs of metals such as iron, aluminium, alloys thereof, brass and bronze, and the prior art is replete with profound experience and knowledge.
For example, it is well known that extreme pressure agents and anti-wear agents which contain elements such as phosphorus and sulphur are effective against friction and wear for pairs of metals, and that these additives form a skin by positively giving rise to a chemical reaction with the surface of the metals, thereby exhibiting functions such as reducing friction and wear and preventing seizing. This prior art has been widely applied in engine oils and gear oils as well as in high performance industrial lubricating oils and greases.
However, notwithstanding the fact that the history of lubrication technology for different materials, such as resin pairs or resins and metals, has been short, the use of these has, as mentioned above, been expanding and diversifying in recent years, but the situation at present is such that no art has yet been offered that completely satisfies the various requirements regarding their lubricating greases.
For example, if the techniques using the phosphorus or sulphur based additives which are effective against friction and wear of metal pairs are applied to the lubrication points of, for example, resin pairs or resin and metal, virtually no effect in reducing friction such as is obtained for metal pairs is obtained. On the contrary, cases where the friction and anti-wear performance may deteriorate and the life of the machines parts may be shortened are by no means rare.
This is considered, in the case of resins, to be because, compared with metals, their surface activity is very weak, so that there is virtually no reaction with organic based additives such as phosphorus or sulphur based additives on the sliding surfaces, and given that adsorption is also weak, the effect on friction and wear is poor, and so the action of reducing friction is weak. Also, if they are used in environments where temperatures inevitably rise, the active phosphorus and sulphur in these additives may permeate inside the resin parts, which may give rise to perverse actions such as the occurrence of cracks or brittleness, or even the promotion of friction and wear.
In order to improve the lubrication state between the aforementioned resin pairs or between a resin and a different material such as a metal, a grease for use in resin lubrication has been proposed (Japanese Laid-open Patent H08-209167 (1996)) where an adequate oil film is maintained for lubrication between metal and resin by incorporating a fatty acid containing hydroxyl groups, or a fatty acid ester of a polyhydric alcohol, in a grease in which the main constituents are a thickener and a base oil, and also where the occurrence of torque fluctuations is inhibited. In addition, a technique has been disclosed (Japanese Laid-open Patent 2001-89778) to reduce wear of the resin parts used under rigorous lubrication conditions, and to give excellent anti-wear properties, by incorporating a polytetrafluoroethylene fine powder in a grease. In addition, a technique has been proposed wherein a lubricating grease containing a polyolefin wax of average molecular weight 900 to 10,000 in a grease which contains a base oil and a thickener manifests low friction on resin lubrication areas and contributes to an improvement in the efficiency of parts having resin lubrication areas (Japanese Laid-open Patent H09-194867 (1997)). And a technique has been disclosed which has superior resin wear resistance by virtue of a grease for use with resins containing as the thickening agent a metal soap thickener, a complex metal soap thickener or a polyurea together with a compound having a laminar structure (Japanese Laid-open Patent 2008-31416). Further improvements may be expected.
The present invention is intended to obtain a grease composition for use in resin lubrication such that friction is alleviated and good lubricity is obtained at lubrication points where at least one element of a pair such as resin and resin or resin and a different material such as metal is constituted of a resin material and rolling or sliding occurs.
Having carried out research and investigations on the theory of the surface chemistry of the lubrication behaviour of resins, the inventors have discovered that the very weak electrical forces, occurring on the surfaces of a resin and its opposing member, in cases such as resin and resin or resin and a different material such as a metal, interact with certain kinds of fatty acid amine salts added to greases, and further that these additives exhibit a binding action with the grease, so that it is possible to create a lubricating film more reliably on the surfaces of the resin and the material opposing the resin and thus obtain a reduction in wear and better lubricating qualities. They have thus arrived at this invention.